The present invention relates to a drum-type washing machine having a drum rotatable about a substantially horizontal axis. In particular, the present invention relates to a washing machine where an outer tub with a drum placed inside is oscillatably held by elastic members. The drum-type washing machine can be used not only for extracting water but also for extracting other liquids such as petroleum solvents from the laundry.
In general, a drum-type washing machine has a cylindrical basket drum rotatable about a horizontal axis. When the drum is rotated at high speed with the wet laundry loaded therein, the water held by the laundry is extracted and scattered by a centrifugal force. One problem concerning such centrifugal extraction is that abnormal vibration and/or noise arises when the drum is rotated at high speed if the mass distribution around the rotation axis is unbalanced as a result of uneven distribution of the laundry on the inner circumferential wall of the drum.
Regarding the problem, various methods of balancing the load in the initial stage of the extracting process have been proposed. By one method, the eccentric load is reduced by evenly scattering the laundry in the circumferential direction of the drum. By another method, a weighing member having a fixed or variable weight is attached to a part of the drum so that the laundry and the weighing member are balanced for reducing the eccentric load. By those conventional washing machines, first the laundry is appropriately distributed or gathered on the circumferential wall of the drum to make the eccentric load of the drum as a whole smaller than a preset amount, and then the drum speed is raised to a high speed for a centrifugal extraction.
The conventional methods concern the load balance in the circumferential direction of the drum, i.e. the load balance around the rotation axis. The methods, however, do not concern the load balance along the rotation axis, i.e. in the direction of the depth of the cylindrical drum. In the drum-type washing machine, the drum is supported by a cantilevered structure where a horizontal rotation shaft fixed to the rear wall of the drum is rotatably held by a bearing member. By such a structure, even when the amount of the eccentric load in the circumferential direction is the same, if the eccentric load is located in the front part of the drum, the load that works on the bearing member in the high-speed extracting process is greater due to the larger distance between the bearing member and the eccentric load. According to the inventors"" study concerning a washing machine where an outer tub with the drum placed inside is suspended by elastic members such as springs for absorbing oscillations, the oscillation of the outer tub becomes greater as the distance between the eccentric load and the bearing member becomes greater.
For addressing the above problems, one object of the present invention is to propose a drum-type washing machine constructed taking account of not only the magnitude of the eccentric load in the circumferential direction of the drum but also the position of the eccentric load in the axial direction of the drum, thus suppressing vibration or noise which is likely to occur when the drum is rotated at high speed.
Thus, in a drum-type washing machine wherein a shaft is rotatably held by a bearing member provided in an outer tub, a drum having a substantially cylindrical circumferential wall is fixed to an end of the shaft, and the drum is driven via the shaft to rotate about a central axis of the circumferential wall at high speed for extracting liquid from the laundry loaded in the drum, the drum-type washing machine according to the present invention is characterized in that the drum is placed so that the central axis of the drum is inclined downwards to a shaft-fixing end of the drum where the shaft is fixed, and a controller controls the rotation of the drum so that the laundry is moved toward the shaft-fixing end in the initial stage of the extracting process.
According to the present invention, the drum is postured so that the central axis is inclined downwards to the shaft-fixing end (or the rear end) of the drum. Therefore, when the laundry is agitated by, for example, rotating the drum, the laundry gradually moves along the inclined circumferential wall and finally comes close to the rear end of the drum. In this state, when the drum speed is raised higher than a specific speed where the centrifugal force and the gravitational force acting on the laundry are balanced (the speed is referred to as the xe2x80x9cequilibrium speedxe2x80x9d hereinafter), it is highly probable that the eccentric load, which is caused by an uneven distribution of the laundry on the circumferential wall of the drum, is located close to the shaft-fixing end. Thus, since the distance between the eccentric load and the bearing member is relatively small, the load that works on the bearing member during the high-speed rotation of the drum is relatively small, so that the wear or damage of the bearing member is decreased. Also, oscillation or vibration of the drum and the outer tub is suppressed while the drum is rotated at high speed.
For the purpose of gradually moving the laundry in the drum along the inclined circumferential wall, the drum may preferably be rotated at a speed slightly lower than the equilibrium speed. By this process, part of the laundry located close to the central axis is preferably scattered in the circumferential direction of the drum while being moved toward the rear end of the drum.
In a preferable mode of the present invention, the washing machine includes an eccentric load detector for detecting the magnitude or an index of the magnitude of the eccentric load due to an uneven distribution of the laundry around the central axis. In an embodiment of the invention, the eccentric load detector is constructed so that the eccentric load is detected based on a torque current component contained in a current supplied to the motor for rotating the drum under the condition that the drum is rotated at a preset speed. When the load around the central axis of the drum is unbalanced, the load torque changes within one rotation period of the drum, and the torque current component accordingly changes. The change corresponds to the magnitude and the position of the eccentric load in the circumferential direction of the drum. Thus, the magnitude of the eccentric load, or amount of eccentricity, can be detected from the change.
The washing machine having the above-described eccentric load detector may be constructed so that whether it is allowable to further raise the speed of the drum to carry out the extracting operation is determined by comparing the magnitude or index of the magnitude of the eccentric load to a preset reference value. By this construction, the drum is allowed to speed up only when the magnitude of vibration arising in the extracting process is expected to be below a preset allowable level. Thus, the vibration is assuredly suppressed in the extracting process.
By the washing machine according to the present invention, the eccentric load, if any, is assuredly located close to the bearing member, so that the reference value for determining whether or not to carry out the extracting process may be set greater than in the case where the eccentric load is far from the bearing member. This means that the allowable level of the amount of eccentricity is substantially greater. Therefore, the process of correcting the balance of the laundry, which conventionally requires a considerably long time, can be completed in a shorter time period.
The process of correcting the balance of the laundry in the initial stage of the extracting process may be such that the drum is simply rotated at a speed slightly lower than the equilibrium speed. Besides, in a preferable mode of the present invention, the washing machine is constructed so that, in the process of correcting the balance of the laundry, the speed of the drum is controlled by a method including steps of rotating the drum at a speed slightly higher than the equilibrium speed and temporarily reducing the speed to be lower than the equilibrium speed when the eccentric load rotating with the drum arrives at the top of the drum.
By the above construction, part of the gathered laundry causing the eccentric load falls off the circumferential wall of the drum due to the speed reduction, whereby the eccentric load is reduced effectively. In addition, since the drum is inclined, the part of the laundry comes closer to the bearing member when it falls onto the bottom of the drum. Thus, the above speed control is advantageous to the movement of the eccentric load toward the shaft-fixing end of the drum.